Orangutans survive only in the dwindling tropical rainforests of Borneo and northern Sumatra, being dependent on the forest for food and nesting sites. Orangutan populations are seriously affected when their forest is destroyed or logged, not least because they are often killed for meat or to protect newly planted crops. For example, in the Sebangau swamp forests of central Borneo, orangutans fled from illegal logging operations, moving into less ideal habitat (Husson et al. 2002). The resulting overcrowding led to an increased death rate among young orangutans, and fewer births amongst females. When the forest started to regenerate, the orangutans were able to return. In Malaysia, the Kinabatangan Orangutan Conservation Project has studied the effects of the transformation wrought by logging on dipterocarp forests. The removal of most large trees means that the heavy adult male orangutans were forced to move along the ground, increasing their vulnerability, but on the other hand, the invasion of the logged forest by vines and pioneer species soon resulted in an increased abundance of fruit (Ancrenaz et al. 2005). If they are not killed in the process, orangutans in these habitats can survive selective logging. Evidence from Ketambe and Gunung Leuser in Sumatra suggests that the ability of these forests to support orangutans initially declines with selective logging, but can recover over time. Over Borneo and Sumatra as a whole, illegal logging has led to huge declines in orangutans and other wildlife. Where forests are converted to plantations of oil palm (Elaeis guineensis) or other crops, the consequences are even more serious, with many orangutans starving.

Like all great apes, orangutans have long lifetimes, long “childhoods” and relatively low reproductive rates, which makes it difficult for them to recover when large numbers are killed. Recent estimates suggest that there are 45 000 to 69 000 Bornean orangutans and only 7 300 Sumatran orangutans remaining in the wild (Caldecott & Miles 2005). The Bornean orangutan is classified as Endangered by IUCN (the World Conservation Union), indicating that it has a very high risk of extinction in the wild in the near future. There are at least three subspecies of Bornean orangutans: Pongo pygmaeus pygmaeus (northwest), Pongo pygmaeus wurmbii (central) and Pongo pygmaeus morio (northeast) (Figure 1). The central Bornean orangutan is the largest, followed by the northwest subspecies, and the northeast subspecies is the smallest.

The Sumatran orangutan is classified as Critically Endangered by IUCN, indicating that it has an extremely high risk of extinction in the wild in the near future. Since 1900, the number of Sumatran orangutans is thought to have fallen by about 91%, with a rapidly accelerating loss towards the end of the twentieth century (McConkey 2005). As a result of logging, infrastructure development, internal migration and plantation development, Sumatra’s forest area was reduced by 61% between 1985 and 1997. The remaining orangutan population is therefore fragmented, with the core of its range being the Leuser Ecosystem. This conservation area is itself recognised in Indonesian law, and contains the Gunung Leuser National Park, which forms part of the Tropical Rainforest Heritage of Sumatra World Heritage Site.

Orangutan biology

Orangutans are intelligent, strong, large primates, and live a semi-solitary life in the trees. A balanced orangutan diet consists of fruits and seeds, but they are also able to eat foodstuffs such as bark, leaves and insects to survive in times of shortage. Fresh sleeping nests are built from branches and leaves almost every evening.

Sumatran orangutans (Pongo abelii) are only found in Indonesia, and Bornean orangutans (Pongo pygmaeus) only in Indonesia and Malaysia, with occasional males reported as wandering into Brunei Darussalam. The Bornean and Sumatran species have formed separate breeding populations for around one to two million years, differing in genetics, behaviour, diet, life history and morphology (MacKinnon et al. 1996; Delgado & van Schaik 2000, Wich et al. 2004; McConkey 2005; Wich et al. 2006a, b; Taylor 2006). Neither species is territorial, but fully developed adult males tend to avoid one another, and occasionally fight if they do meet.

There is a serious need for conservation action on both islands, because even within these formally protected areas, orangutans are under pressure. Priority populations for conservation action (Figure 1, 2) have been identified by scientists working with the Great Apes Survival Project (GRASP). The goal is to retain viable populations of both orangutan species and all three Bornean subspecies in their natural habitats wherever they exist, conserving their genetic, cultural and ecological diversity.